To enhance the yield in fabricating integrated circuits from semi-conductor wafers by ion beam doping, it is important to achieve doping uniformity across the wafer surface. One factor that affects doping uniformity is the uniformity of impact angle between the ion beam and the silicon wafer during ion implantation. In prior art medium current implanters, an ion beam is electrostatically deflected from an original trajectory by several degrees to sweep across the silicon wafer and dope the silicon wafer in a controlled, uniform manner. When the diameter of the silicon wafer is on the order of 10 centimeters, such deflection produces small angular deviations of impact angle. As the size of the silicon wafer increases, however, the deviation in impact angle from the center to the outer edge of the wafer also increases.
To sweep across larger diameter wafers, the medium current ion beam trajectory has increased, causing problems in beam transmission. Increasingly, even modest angle variations have become unacceptable for these implanters. Various techniques have been tried to avoid angle variation caused by beam deflection. The most common procedure is to deflect the beam back after its original deflection before it reaches the region of impact with the silicon wafer.
Examples of prior art patents addressing the variation in impact angle are U.S. Pat. No. 4,794,305 to Matsukawa; U.S. Pat. No. 4,276,477 to Enge; U.S. Pat. No. 4,687,936 to Mcintyre et al.; and U.S. Pat. No. 4,922,106 to Berriam et al.
U.S. Patent No. 5,091,655 to Dykstra et al. discloses an ion beam implantation system where an ion beam is controllably deflected from an initial trajectory as it passes through spaced parallel plates. Once deflected, the ion beam enters an accelerator that both redeffects the once-deflected ion beam and accelerates the ions within the beam to a final desired energy. Ions within the beam exiting the accelerator follow trajectories that impact the workpiece at controlled impact angles.
As a beam is deflected electrostatically, it is also focused in the plane of deflection. The focusing strength is proportional to the square of the angle of deflection. As a result, the manner in which the beam is imaged varies through the angles of deflection. As the angle of deflection becomes larger, the focusing effect also becomes larger. The consequence of this focusing effect is that the beam size and shape at the target varies as a function of position, making doping uniformity difficult to achieve even though the impact angle of ions with the workpiece is well controlled.